The effectiveness of the Gat~n Lake fresh-water barrier to fish migration of the Panama Canal was examined, based on literature records and recent collections. Six species have migrated to the Pacific and three to the Atlantic Oceans. New records include: Hypleurochilus aequipinnis, Barbulifer ceuthoecus and Oostethus lineatus of Atlantic origin, and Gna~anodon speciosus of Pacific origin. The majority of Atlantic migrants are known in the Pacific only from the Miraflores Third Lock, a unique ectogenic meromictic lake attached to the Pacific entrance of the canal. The hydrography and biota (including Atlantic algal and crustacean species previously unknown as canal migrants) of the lake are discussed. It is presumed that certain fish migrants transited the canal by associating with fouling material on the underside of ships. The euryhaline species H. aequipinnis, Lupinoblennius dispar, Lophogobius cyprinoides and Omobranchus punctatus survived in freshwater (O.O~S) for periods longer than required for ship transit of the canal (ca. 8 h). The stenohaline migrant Gobiosoma nudum died after 2 h in freshwater, but survived more than 50 h at 2.5~S. Plans to increase Panama Canal ship transits through the pumping of seawater into Gat~n Lake might remove the biological barrier and allow the migration of euryhaline and stenohaline species. We find those plans unwise, and the potential consequences dangerous.